1. Field of the Invention
The present invention generally relates to communication systems, and more particularly, to enhance position location using a global navigation satellite system.
2. Background of the Invention
It is often desirable, and sometimes necessary, to know the location of a mobile station, (e.g., a cellular phone). The terms “location” and “position” are synonymous and are used interchangeably herein. For example, a user may utilize a mobile station (MS) to browse through a website and may click on location sensitive content. The location of the mobile station may then be determined and used to provide appropriate content to the user. There are many other scenarios in which knowledge of the location of the mobile station is useful or necessary. For example, the FCC's 911 mandate requires carriers to provide enhanced 911 services including geographically locating a mobile station making a 911 emergency services call. The mobile station may be provisioned such that it can obtain location services from a home network and also while roaming in a visited network. The mobile station may communicate with various network entities in the home network in order to determine the location of the mobile station whenever needed.
There are many different types of technologies employed in calculating the location of mobile stations in wireless networks with various levels of success and accuracy. Network based methods include angle of arrival (AOA) using at least two towers, time difference of arrival (TDOA) using multilateration, and location signature using RF fingerprinting to match RF patterns that mobile stations exhibit at known locations. Various mobile station based methods incorporate GPS, Advanced Forward Link Trilateration (A-FLT), Timing Advance/Network Measurement Report (TA/NMR) and/or Enhanced Observed Time Difference (E-OTD).
Another mobile station based method is assisted-GPS (A-GPS), in which a server provides Assistance Data to the mobile station in order for it to have a low Time to First Fix (TTFF), to permit weak signal acquisition, and to optimize mobile station battery use. A-GPS is used as a location technology in isolation or hybridized with other positioning technologies that provide range-like measurements. An A-GPS server provides data to a wireless mobile station that is specific to the approximate location of a mobile station. The Assistance Data helps the mobile station lock onto satellites quickly, and potentially allows the handset to lock onto weak signals. The mobile station then performs the position calculation or optionally returns the measured code phases to the server to do the calculation. The A-GPS server can make use of additional information such as round-trip timing measurements from a cellular base station to the mobile station in order to calculate a location where it may otherwise not be possible; for example when there are not enough GPS satellites visible.
Advances in satellite-based global positioning system (GPS), timing advance (TA), and terrestrial-based Enhanced Observed Time Difference (E-OTD) position fixing technologies enable a precise determination of the geographic position (e.g., latitude and longitude) of a mobile station. As geographic location services are deployed within wireless communications networks, such positional information may be stored in network elements and delivered to nodes in the network using signaling messages. Such information may be stored in a Serving Mobile Location Center (SMLC), a Stand-Alone SMLC (SAS), a Position Determining Entity (PDE), a Secure User Plane Location Platform (SLP) and special purpose mobile subscriber location databases.
One example of a special purpose mobile subscriber location database is the SMLC proposed by the 3rd Generation Partnership Project (3GPP). In particular, 3GPP has defined a signaling protocol for communicating mobile subscriber positional information to and from an SMLC. This signaling protocol is referred to as the Radio Resource LCS (Location Services) protocol, denoted RRLP, and defines signaling messages communicated between a mobile station and an SMLC related to a mobile subscriber's location. A detailed description of the RRLP protocol is found in 3GPP TS 44.031 v7.9.0 (2008 June) 3rd Generation Partnership Project; Technical Specification Group GSM Edge Radio Access Network; Location Services (LCS); Mobile Station (MS)-Serving Mobile Location Center (SMLC) Radio Resource LCS Protocol (RRLP) (Release 7).
In addition to the United States Global Positioning System (GPS), other Satellite Positioning Systems (SPS), such as the Russian GLONASS system or the proposed European Galileo System may also be used for position location of a mobile station. However, each of the systems operates according to different specifications.
One weakness of a satellite based position location system is the time taken to acquire an accurate position fix. Typically, position accuracy is traded off for acquisition speed and visa versa. That is, a more accurate fix takes more time. Accordingly, there is a need for a communication system, including a global navigation satellite system (GNSS), which can determine a position location for a mobile station based on satellite signals sent from two or more satellites to provide further efficiencies and advantages for position location including enhanced accuracy. A need exists to enhance accuracy while not detrimentally impacting the acquisition speed or a final acquisition time of acquiring a position fix of a mobile station, for example, during an emergency services (ES) call or value added services (VAS) session.